U-shaped mechanical vibrator

ABSTRACT

A U-shaped mechanical vibrator having a pair of strip-like vibratory reeds of substantially the same configuration, and a base portion coupling together the pair of vibratory reeds at one end as a unitary structure, the width of each reed being selected greater than the thickness thereof, the vibratory reeds being arranged in a single plane including their surfaces in the widthwise direction in parallel and side-by-side relation, and the pair of vibratory reeds vibrating in anti-phase relation to each other at right angles to the single plane.

United States Patent Tanaka et a1.

[54] U-SHAPED MECHANICAL VIBRATOR inventors:

Tetsuro Tanaka, Kyoto; Kiyoshi Bansho,

Tokyo, both of Japan Assignee:

Filed:

Appl. No.:

Shigeru Kakubari, Tokyo, Japan 0 Nov. 10, 1970 Related US. Application Data Continuation of Ser. No. 754,416, Aug. 21, 1968,

abandoned.

Foreign Application Priority Data Aug. 24, 1967 Japan ..42/54387 Aug. 24, 1967 Japan... ....42/54388 Aug. 24, 1967 Japan... ....42/54389 Aug. 24, 1967 Japan ..42/54390' US. Cl. ..333/71, 58/23 TF, 331/116 M,

331/156 Int. Cl. ..H03b 5/36 Field ofSearch ..58/23;331/l16, 156,71

[451 Apr. 25, 1972 [56] References Cited UNITED STATES PATENTS 3,007,111 10/1961 Umile et a1. ..333/71 3,024,429 3/1962 Cavalieri et a1... ...33 l/l56 3,361,994 1/1968 Takahashi ..331/71 3,462,939 8/1969 Tanaka et a1. ..58/23 Primary Examiner-John Kominski AtlorneyHill, Sherman, Meroni, Gross & Simpson [57] ABSTRACT A U-shaped mechanical vibrator having a pair of strip-like vibratory reeds of substantially the same configuration, and a base portion coupling together the pair of vibratory reeds at one end as a unitary structure, the width of each reed being selected greater than the thickness thereof, the vibratory reeds being arranged in a single plane including their surfaces in the widthwise direction in parallel and side-by-side relation, and the pair of vibratory reeds vibrating in anti-phase relation to each other at right angles to the single plane.

5 Claims, 27 Drawing Figures PATENTEDAPRZS m2 3,659,230

sum as or 14 PATENTED APR 2 5 m2 sum 110F 1 PATENTEIJAPR 25 I972 sum 1n 0F 14 BACKGROUND OF THE INVENTION I. Field of the Invention This invention relates to a vibrator for use in oscillators, mechanical filters or the like, and more particularly to a vibrator which is small in size, easy to manufacture and suitable for mass production.

2. Description of the Prior Art In the prior art the so-called tuning fork has been proposed as a vibrator but fabrication of such a conventional tuning fork involves an appreciable amount of time and high precision cannot be expected so that the prior art tuning fork is not suitable for mass production. Further, miniaturization is very difficult.

SUMMARY OF THE INVENTION The principal object of this invention resides in the provi sion of a novel mechanical vibrator which is free from the drawbacks experienced in the prior art.

The mechanical vibrator of this invention can be produced by punching process or etching of a thin sheet metal, and hence is easy to manufacture, high in precision and suited for mass production. Further, the present invention allows ease in the production of extremely miniaturized vibrators.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view showing a conventional tuning fork;

FIG. 2 is a plan view showing one example of a planar tuning fork type mechanical vibrator produced according to this invention;

FIG. 3 is a side view of the vibrator depicted in FIG. 2;

FIG. 4 is a perspective view showing the manner in which the vibrator of FIG. 2 vibrates;

FIG. 5 is a graph showing loss-frequency characteristics relative to the position of electromechanical transducer elements;

FIG. 6 is a graph showing loss-frequency characteristics relative to the size of the electromechanical transducer elements;

FIG. 7 is a connection diagram illustrating one example of an oscillator circuit employing the vibrator of this invention;

FIG. 8 is a graph showing its frequency variation rate relative to temperature change;

FIG. 9 is a plan view illustrating, by way of example,-one process for the manufacture of the vibrator of this invention;

FIG. 10 is a plan view showing still another example of the vibrator of this invention;

FIG. 11 is a side view of the vibrator depicted in FIG. 10;

' FIG. 12 is a plan view illustrating one example of a planar compound vibrator consisting of two vibrators of this invention assembled in side-by-side relation;

FIG. 13 is a side view of the planar compound vibrator exemplified in FIG. 12;

FIG. 14 is a connection diagram illustrating one example of the planar compound vibrator as applied to an oscillator;

FIG. 15 is a graph showing its frequency variation rate relative to temperature change;

FIGS. 16A and 16B are side views respectively illustrating other examples of the planar compound vibrator of this invention;

FIG. 17 is a plan view showing one example of a filter employing two planar vibrators of this invention;

FIG. 18 is a side view of the filter shown in FIG. 17;

FIG. 19is a graph showing loss frequency characteristic curves of the filter of FIG. 17 with the coupling degree of its vibrators being as a parameter;

FIG. 20 is a graphical representation of the relationship of the coupling degree to frequency deviation;

FIG. 21 is a schematic diagram showing the connections of a signal source, the filter and output terminals;

FIG. 22 is a graph showing loss-frequency characteristic curves with an external resistance being as a parameter;

FIG. 23 is a plan view illustrating another example of the filter of this invention;

FIG. 24 is a perspective view illustrating still another example of the filter of this invention;

FIG. 25 is a plan view illustrating one example of a frequency selector device employing a plurality of vibrators of this invention; and

FIG. 26 is a side view of the frequency selector device depicted in FIG. 25.

DESCRIPTION OF PREFERRED EMBODIMENTS In FIG. 1 there is schematically illustrated one example of a conventional mechanical vibrator, commonly referred to as a tuning fork, which consists of a pair of vibratory reeds la and lb arranged in opposing and predetermined spaced relation and a common base portion 2 interconnecting the reeds. Such a tuning fork is usually produced by machining of a metal block. However, the machining process presents a problem as it requires an appreciable amount of time and introduces a dif ficulty in obtaining the tuning fork with high precision and hence the prior art tuning fork is not suited for mass production with uniform characteristics. In particular, this imposes a severe limitation on the construction of small-sized tuning forks.

A detailed description will hereinafter be described in connection with a U-shaped mechanical vibrator of this invention which is free from the drawbacks encountered in the prior art.

In accordance with this invention the U-shaped mechanical vibrator consists of a pair of vibratory reeds 3a and 3b formed of resilient metal sheets in substantially the same configuration and arranged to extend in parallel with each other in the same plane, while being spaced a predetermined distance d,, and a common plate-like base portion 4 joined contiguously to the vibratory reeds 3a and 3b and lying in the same plane as the reeds, as illustrated in FIGS. 2 and 3. On the vibratory reeds 3a and 3b there are fixedly mounted in close proximity to the base portion 4 electromechanical transducer elements such as piezoelectric elements 5a and 5b of, for example, PZT (zircon lead titanate). Further, one portion of the base portion 4 remote from the vibratory reeds 3a and 3b is extended and is bent substantially at right angles to the surface of the reeds 3a and 3b to provide a coupling portion 6. The free end portion of the coupling portion 6' is bent substantially at right angles to be parallel with the reeds 3a and 3b, thus providing a mounting portion 7. The mounting portion 7 is secured to a base plate 6 by means of, for example, a screw 8, by which the U-shaped vibrator is mounted on the base plate 6. The U- shaped vibrator may be formed, for example, by punching a thin metal sheet to have the vibratory reeds 3a and 3b, the base portion 4, the coupling portion 6' and the mounting portion 7 as a unitary structure. The vibrator may be made of a resilient material having small coefficient of thermal expansion such, for example, as ELINVAR. With such a vibrator (hereinafter referred to as a planar tuning fork) it has been ascertained that the vibratory reeds 3a and 3b vibrate opposite in phase in a direction at right angles to their surfaces (in this case the width W of the vibratory reeds 3a and 3b is selected well greater than the thickness t thereof). Namely, the vibratory reeds 3a and 3b vibrate in such a mode that they first swing away from their common reference plane in opposite directions and then back to the reference plane, as depicted in FIG. 4, in which manner the vibratory reeds 3a and 3b repeatedly vibrate.

The resonance frequency fof the planar tuning fork is given by the following equation:

the tuning fork and is l.875l in the case of basic vibration, namely m being equal to I, l is the length of the vibratory reeds 3a and 3b in their longitudinal direction (refer to FIG. 2), t their thickness (refer to FIG. 3) p the density of their material and E its Youngs modulus.

For example, in the case where I is 18.5 mm., t is 0.5 mm., the width W of the vibratory reeds 3a and 3b is 1.8 mm. and the distance d between the vibratory reeds is 1 mm., the resonance frequency f of the tuning fork is 1015 c/s from the above equation (1).

Now, a discussion will be made in connection with the influence exerted on the vibration of the vibratory reeds 3a and 3b by the position of the piezoelectric elements 5a and 5b. With the distance d from the piezoelectric elements 5a and 5b to a demarcation line 9 between the vibratory reeds 3a and 3b and the base portion 4 being 1 mm., and 1 mm., the elements 5a and 5b each being formed of PZT to have a length L of 3.0 mm., a width W of 1.7 mm. and a thickness of 0.25 mm., the insertion loss vs. frequency characteristics are respectively given as indicated by curves l0, 1 1 and 12 in FIG. 5 in which the ordinate represents loss in dB and the abscissa frequency in c/s and d is employed as a parameter. As will be apparent from the graph, when d 1 mm. the resonance frequency f is 1015.5 c/s nearly equal to the aforementioned calculated value and when d 0 and -1 mm. the resonance frequencies are respectively as high as 1041.5 c/s and 1044.5 c/s. This is considered to result from the fact that the piezoelectric elements 5a and 5b act as stifiness on the vibratory reeds 3a and 3b, not as mass. The Qs of the planar tuning fork in the above three cases are l 128 in the case of d being 1 mm., 1160 in the case ofd being 0 and 1158 in the case ofd being -1 mm., and the Q in the first case is a little lower than the others. This implies that the Q lowers as the piezo-electric elements approach the side of the vibratory reeds 3a and 3b of large amplitude, that is, their free ends. Consequently, the insertion loss is most minimized with d being 1 mm. The ratio of an output voltage to an input voltage, that is, the coupling factor is a little greater when d 1 mm. The foregoing numerical values are given in the following table 1.

FIG. 6 is a graph showing the influence exerted upon the insertion loss by changing the size of the piezoelectric elements 50 and 5b, in which the ordinate represents the loss in dB and the abscissa frequency in c/s. In the figure the curve 13 indicates a case of the piezoelectric elements each having a length of 3 mm. and a width of 1.7 mm. and the curve 14 a case of the elements having a size of a length of 1.3 mm. and a width of 1.2 mm. It appears from the graph that the resonance frequencies are approximately equal to 1015.5 c/s and Qs are also substantially equal to 1128 but that the insertion losses are 0.7 dB and 3 dB. Further, the coupling factors are 0.027 and 0.025, namely the smaller piezoelectric elements lightly lower the coupling factor.

It is preferred that the distance d between the vibratory reeds 3a and 3b be smaller than the width W, of each reed,-for example, less than one-half thereof. This is because of the fact that with the distance d being greater than the width W of the reeds 3a and 3b, torsion or twist is yielded in the base portion 4, that is, the coupling portion of the reeds which is likely to cause a variation in the resonance frequency. In addition, the width W, of the vibratory reeds is selected smaller than the length 1 of the base portion 4 in the lengthwise direction of the vibratory reeds and the length I, is selected, for example,

more than 1.5 times as great as the width W The reason is that the length l, smaller than the width W, causes an increase in vibration rendered to the mounting portion 7 through the coupling portion 6' from the base portion 4.

Where the piezoelectric elements 5a and 5b are deposited on the same side of the vibratory reeds 3a and 3b, an input signal fed to either one of the piezoelectric elements and an output signal obtained from the other piezoelectric element are opposite in phase. Consequently, an output signal in phase with the input signal may be obtained by the use of piezoelectric elements of opposite polarities or by depositing the piezoelectric elements on different surfaces of the vibratory reeds.

FIG. 7 illustrates one example of a self-excited oscillator employing the planar tuning fork of this invention described above, which may be-provided by the same connections as a self-excited oscillator using a conventional tuning fork except the use of the planar tuning fork of the present invention, as depicted in the figure. Namely, an amplifier 21 consisting of transistors 19 and 20 of cascade connection is provided and connections are made such that one portion of the output from the output terminal of the amplifier 21 is applied to the piezoelectric element, for example 5a of the vibratory reed 3a of the planar tuning fork and an output obtained by the other piezoelectric element 512 of the vibratory reed 3b is fed to the input side of the amplifier 21, that is, to the base of the transistor 19, thus providing an oscillator having an oscillation frequency determined by the resonance frequency of the planar tuning fork. In FIG. 8 there is depicted the frequency variation rate Af/f relative to temperature change C) in the above case, the ordinate representing the frequency variation and the abscissa temperature. This graph was obtained in the case where only the tuning fork was subjected to temperature change. From the graph it appears that the frequency variation is 1 X 10" deg. in a range of about 19 to 59 C, which indicates an excellent characteristic.

Although the foregoing has stated that the planar tuning fork of this invention may be produced by press work (punching process), it may also be made of one base plate by means of chemical etching techniques. That is, grooves 16 are formed in a thin sheet 15 of ELINVAR by chemical etching in a manner to leave the vibratory reeds 3a and 3b, the base portion 4 and the coupling portion 6, as shown in FIG. 9. In this case, it is preferred to form a groove 17 by chemical etching at a distance from the coupling portion 6 on the opposite side from the vibratory reeds. With photoetching techniques used in the manufacture of semiconductor devices, smalledsized tuning forks can be produced with high precision, and the planar tuning fork thus produced is suitable for use with, for example, semiconductor integrated circuit devices.

In the foregoing example the piezoelectric elements are used as electromechanical transducer elements but may be substituted with electromagnetic transducer elements. Namely, instead of mounting the piezoelectric elements 5a and 5b on the planar tuning fork, electromagnetic units 18a and 18b each consisting of a core and a coil wound thereon are disposed opposite the vibratory reeds 3a and 3b in the vicinity of the free'ends where their amplitude of vibration is great. With an exciting signal being fed to, for example, the electromagnetic unit 18a, the vibratory reed 3a is driven to vibrate and the vibratory reed 3b is thereby driven through the base portion 4, by which an electric signal of the same frequency as the signal fed to the unit 18a is obtained from the electromagnetic unit 18b.

As has been described in the foregoing, this invention enables mass production of planar tuning forks of high precision and uniform characteristics without requiring such a troublesome machining of a metal block.

In FIG. 12 there is illustrated another example of this invention in which a plurality of, for example, two planar tuning forks such as depicted in FIG. 2 are jointed together at their base'portions with'their surfaces being flush with each other. A detailed description will liereinbelow be given of this example. Reference numerals 101 and 102 indicate two planar tuning forks, which are assembled together in the following manner. That is, vibratory reeds 103a, 1021b and 103a,, 103b are disposed substantially in parallel at moderate intervals with their planar surfaces being substantially flush with one another, and coupling portions 106 and 106 extending from the base portions 104 and 104 on the side remote from the vibratory reeds in parallel relation thereto are secured to a common support 113, thereby providing an assembly of the tuning forks. In this case the base portions 104 and 104 of the planar tuning forks 101 and 102 are jointed together through a joint portion 109. Reference numerals 105a, 105b, 105a, and 105b designate piezoelectric elements fixed mounted on the vibratory reeds of the planar tuning forks, which are identical with those exemplified in FIG. 2. The planar tuning forks thus assembled together are mounted on a base plate 106 by clamping their common support 113 onto the base plate 106 by means of a screw 116 inserted through a hole 115 bored in the support 1 13. It is a matter of course that such a compound tuning fork consisting of the two planar tuning forks 101 and 102 depicted in FIG. 12 may be produced from a sheet metal by means of punching or etching in the same manner as that described above with FIG. 2.

The resonance frequency of each tuning fork is determined by the equation (1) as previously described but the frequencies of the two planar tuning forks are rendered difierent from each other by selecting the length of the vibratory reeds of either one of the tuning forks to be smaller or greater than that of the reeds of the other. In FIG. 12 the vibratory reeds 103a and 10317 of the tuning fork 102 are shorter than those 103a and 103b of the other tuning fork 101. It is preferred that the vibratory reeds of the same tuning forks, that is, 103a and 103b or 103a and 103b are identical in shape with each other.

Assuming that the widths of the vibratory reeds of the planar tuning forks 101 and 102 are taken as a, and 11 the distance between the vibratory reeds 103a and l03b is 11,, the distance between the reeds 103a and 103b, is b and the lengths of the reeds 103a, 103b and 103a,, 103b, are I, and 1 respectively, a,, b,, 11 and b are selected such that a b and a b,. This is because of the fact that b or b exceeding 0 or 0 produce torsion in the base portion 104 or 104 to cause a change in the resonance frequency. Further, if the lengths of the base portions 104 and 104 in the lengthwise direction of the vibratory reeds are taken as c, and 0 (c, c 2 in the figure), c and 0 are selected greater than the widths a and a of the vibratory reeds. With 0 being smaller than a or a vibration of the baseportion 104 or 104 in the, direction of the coupling portions 106' and 106 increases, which is undesirable as set forth above.

In order to provide the coupling portion 109, a slit 116 is formed in the jointed portion of the base portions 104 and 104, on the side of the vibratory reeds, in which case the depth W of the slit is selected great enough to permit the tuning forks 101 and 102 to function independently from each other.

For instance, a typical size is such that a =a 1.8 mm., 12 b 1.0 mm., W= 1.6 mm., the thickness t, of the vibratory reeds 103a, 1013b and 103a,, 103b =0.5 mm. and the distance d between the adjacent vibratory reeds 10311 and 103a 1.0 mm. In such a case, the resonance frequencies f and f of the planar tuning forks 101 and 102 are 1377 c/s and 1299 c/s respectively. To electrically drive the tuning forks 101 and 102 to obtain electric signals therefrom, piezoelectric elements 105a, 105b and 105a,, 10512, of, for example, PZT may be deposited by an adhesive binder on the vibratory reeds 103a, 103b and 103a and 10312 on the side of the base portions 104 and 104 In the example shown in FIG. 12 the planar tuning forks 101 and 102 are formed as a unitary structure but each of them performs the function of substantially an independent tuning fork. Consequently, it is possible that separate oscillators having the tuning forks as the reference frequency sources are provided and adapted to obtain an output of a frequency corresponding to a difference in their oscillation outputs, so that an oscillation output of low frequency can be obtained with relatively small tuning forks. FIG. 14 is a connection diagram illustrating one example of such construction. In the figure reference numeral 124 indicates an amplifier consisting of transistors 119a and 119b, the transistor 119a having its collector connected to a power source 120 and its emitter grounded through a resistor 121 and connected to the base of the transistor 11% and the transistor 11% having its collector connected to the power source 120 through a resistor 122 and its emitter grounded through a resistor 123. Reference numeral 125 designates an oscillator circuit having incorporated therein the amplifier 124. Namely, the input end of the amplifier 124, that is, the base of the transistor 1190 is connected to, for example, the piezoelectric element l05b of the vibratory reed 103b of the planar tuning fork 101, and the output side of the amplifier, that is, the collector of the transistor 11% is connected to the piezoelectric element 105a of the vibratory reed 103a, by which the vibratory reed 103a is driven to drive the vibratory reed 103b and the transistor 1 19b is driven by an output of the piezoelectric element l05b of the vibratory reed 103k to permit oscillation of the oscillator circuit 125 at the resonance frequency of the planar tuning fork 101. In a similar manner an amplifier is constituted with transistors 126a and 12612, and the input side of the amplifier 130, that is, the base of the transistor 126a is connected to the piezoelectric element 1051? of the other tuning fork 102 and the output side of the amplifier, that is, the collector of the transistor 12Gb is connected to the piezoelectric element 105a,, thus providing an oscillator circuit 126 oscillating at the resonance frequency of the planar tuning fork 108. Further, these oscillator circuits 125 and 126 are interconnected and their outputs are fed to a frequency converter. In the figure the oscillators 125 and 126 are coupled together by the mechanical coupling of the planar tuning forks 101 and 102 through the coupling portion 109, under which conditions when the oscillator, for example, 126 is main, the oscillation frequency f of the oscillator 126 is amplitude-modulated at the oscillation frequency f of the oscillator 125. Accordingly, the output of the oscillator 126, that is, the collector output of the transistor 12611 is fed through a low-pass filter 131 to an amplifier 133 consisting of a transistor 132 of emitter-grounded connection, from an output terminal 134 of which amplifier can be obtained a signal f f fl, corresponding to a difierence in the oscillation frequencies of the oscillators 125 and 126. It is also possible in this case that the respective outputs of the oscillators 125 and 126, that is, the output of the transistor 1 19b and the collector output of the transistor 12617 are separately applied to the common frequency converter circuit to obtain a beat frequency therebetween. In order to facilitate coupling of the planar tuning forks through the coupling portion 109 for obtaining an amplitude-modulated signal, it is preferred that in the planar tuning fork 102 of the main oscillator the outer vibratory reed l03b (on the opposite side from the planar tuning fork 101) is a drive side and the inner reed 103a a pickup side and that in the other planar tuning fork 101 the inner vibratory reed 103b is a drive side and the outer reed 103a a pickup side. The planar tuning forks 101 and 102 for producing the reference frequencies of the oscillators 125 and 126 for obtaining a beat signal are produced as a unitary structure by punching of a resilient sheet of metal, for example, elinvar. Consequently, the frequency vs. temperature characteristics of the two oscillators are substantially the same and further since a difference frequency is obtained, a beat frequency output remarkedly stable in temperature can be produced. In FIG. 15 there is illustrated frequency change ratio Af/f relative to temperature change in the case where only the planar tuning forks 101 and 102 are subjected to temperature change. It appears from the graph that the temperature coefiicient is substantially zero in a temperature range of 19 to 65 C. In the illustrated example f 1377 c/s,f 1299 c/s andfi, 78 c/s at a temperature of 20 C. Since a signal corresponding to the difference in the oscillation frequency between the two oscillators is taken out as described above, even if the planar tuning forks 101 and 102 are miniaturized, a low-frequency signal can be obtained. In the prior art, a tuning fork oscillating, for instance, at 78 c/s is bulky and is difficult to drive. In the example depicted in FIG. 14 the piezoelectric elements are employed as electric transducer elements but they may be replaced with, for instance, electromagnetic transducer units or electrostatic transducer units, as will hereinbelow be described with FIG. 16. That is, as depicted in FIG. 16A an electromagnetic unit 135 consisting of a magnetic member and a coil wound thereon is disposed opposite the vibratory reed of the planar tuning fork, or fixed electrode 136 is placed as the electrostatic transducer unit in opposed relation to the vibratory reed, as illustrated in FIG. 16B, in which case a high DC voltage source 137 is applied between the electrode 136 and the vibratory reed while at the same time applying or taking out an AC signal. It will be apparent that all or some of the piezoelectric elements 105a, 105b, 105a and 10517, may be substituted with the electromagnetic or electrostatic transducer elements and that all these transducer elements may be used in combination. Although the foregoing description has been made in connection with the use of an assembly of two planar tuning forks, it is possible to use an assembly of three tuning forks, in which case a difference between the vibration frequencies of two tuning forks is first obtained and then a difierence between the resulting difference frequency and the vibration frequency of the remaining tuning fork is obtained. Further, it is easy to produce an oscillator having oscillation frequencies corresponding to the differences in the vibration frequencies of more than four tuning forks.

In FIGS. 17 and 18 planar tuning forks of this invention such as depicted in FIG. 2 are mechanically coupled together in the same manner as in the example shown in FIGS. 12 and 13. The similar parts to those in FIGS. 12 and 13 are identified by the similar reference numerals and no detailed description will be repeated for the sake of brevity. In this case the vibratory reeds 103a, 103b, 103a and l03b of the planar tuning forks 101 and 102 are substantially equal in length l, to one another. As driving and detecting elements of a filter, piezoelectric elements of, for example, PZT are used but in the present example the piezoelectric elements are mounted on the two outer vibratory reeds 103a and 103b, of the tuning forks 101 and 102 in proximity to the base portion 104 and 104 thereof, as indicated by 105a and 105b,.

The resonance frequency f of these planar tuning forks 101 and 102 is given by the aforementioned equation l 1 a i .P

21r Z2 For example, where 1 1.85 cm. and t 0.05 cm., the resonance frequency f is 1015.0 c/s.

Applying an electrical signal to the piezoelectric element 105a of the planar tuning fork 101, the vibratory reed 103a is thereby vibrated, which leads to driving of the vibratory reed l03b in anti-phase relation to the reed 1030, thus rendering the planar tuning fork in its driven condition. This applies vibration to the planar tuning fork 102 through the coupling portion 109 to cause its vibratory reeds 103a and 103b to be driven at the same time, with the result that an electrical signal is taken out from the piezoelectric element l05b of the vibratory reed l03b of the planar tuning fork 102. In such a case, the planar tuning forks 101 and 102 are caused to vibrate only by a signal having a particular frequency equal to their resonance frequency, and they hardly vibrate at. other frequencies. Therefore, a filter having a pass band corresponding to the oscillation frequency of the planar tuning forks can be provided.

By the way, if the width of the vibratory reeds of each of the planar tuning forks 101 and 102 is taken as a and the distance between adjacent vibratory reeds is b, b is selected to be about one half of a. When b is greater than a pseudovibration is caused in the base portions 104 and 104 to shift the resonance frequency. In addition, the length 0 of the base portion 104 or 104 in the lengthwise direction of the vibratory reeds is selected greater than the width a of the vibratory reeds, for example, about 1.5 times as great as the width a. This is because of the fact that when c is smaller than a vibration is much transmitted to the support 113 through the coupling portions 106' and 106,. It is preferred to locate the piezoelectric elements a and 105b, a little further to the free end of the vibratory reeds than the demarcation between the reeds and the base portions 104 and 104,. In this case the resonance frequency of the planar tuning forks becomes approximately equal to the aforementioned equation (1). There is a tendency that shifting of the piezoelectric elements toward the free ends of the vibratory reeds causes their resonance frequencies to deviate higher. Now, the coupling portion 109 will be discussed. If the distance from the coupling portion 109 to the demarcation between the vibratory reeds 103b and 103a and the base portions 104 and 104 is taken as W, a loss characteristic curve with W varying as a parameter is as shown in FIG. 19, in which the ordinate represents loss in dB and the abscissa frequency in c/s. That is, curves 12, 13 and 14 respectively indicate the cases of W being 1.50 mm., 1.56 mm. and 1.62 mm. From the graph it appears that a decrease in W tightens the coupling of the two planar tuning forks 101 and 102 and causes the characteristic curve to be double-humped and widens its pass band, for example, up to about 8 c/s in the graph. In the case of the curve 13 the pass band is approximately 5 c/s. With the lowering of the coupling of the planar tuning forks, that is, with an increase in W, the characteristic curve becomes to be substantially single-humped and its pass band width becomes 2.5 c/s. The depths of the troughs of the curves 12, 13 and 14 are respectively 8 dB, 5 dB and 1 dB. It will be understood from this that the band width can be widened by decreasing W to increase the coupling of the planar tuning forks 101 and 102 and that the band width can be narrowed by increasing W to decrease the coupling. In the above example the values of a, b, t are those previously mentioned and the piezoelectric elements of PZT are employed and are of a size of 3 X 2 mm. Further, it will be seen that the relation between the frequency difference of the peaks of the characteristic curves and W is such that an increase in W causes a linear decrease in the frequency difference as shown in FIG. 20, the abscissa representing W in mm. and the ordinate the frequency difference in c/s.

The filter characteristic of the filter such as depicted in FIGS. 17 and 18 can be improved by input and output resistances. That is, as shown in FIG. 21, a signal is applied to the filter 215 of this invention from a signal source through a resistor 217, namely the signal is fed to the piezoelectric element 105a of the filter 215, and a resistor 218 is connected between the output side or the piezoelectric element 105b, and ground, and output terminals 219 are led out from the both ends of the resistor 218. This provides a maximum output when the resistance value R of the resistor 218 on the output side is 1/21rfc c being the capacitive component of the piezoelectric element PZT. Where the resonance frequency is 1,000 c/s and the capacitive component 0 of PZT is 2,000 PF, R l megohm is a maximum output. In FIG. 22 there is illustrated a graph showing loss frequency characteristic curves obtained with the resistance values of the resistors 217 and 218 being altered, the ordinate representing loss in dB and the abscissa frequency in c/s. The curve 20 in the graph indicates the case where the resistance value R of the resistor 217 is zero and R is infinite, in which case the depth of the trough between peaks of the curve 20 is 6.5 dB and the insertion loss at the peaks is approximately zero. When R 0 and R 1 M0, the insertion loss at the peaks is about 9 dB but the characteristic curve becomes nearly singlehumped and the depth of the trough is 3 dB. Further, when R is 1 M0, the insertion loss is the same as in the above but the characteristic curve becomes further single-humped and the depth of the trough is about 1 dB. The band width does not vary with the resistance value R but lowers from 5 c/s to 4.5 c/s when R is altered from 0 to 1 MO.

As has been described above, this invention permits of fabrication of the tuning fork filter by means of punching a sheet metal and allows ease in mass production of miniature and highly precise filters. The band width can be adjusted by controlling the coupling degree of the coupling portion 109 of the two planar tuning forks 101 and 102.

The filter described above exhibits excellent temperature characteristic such that the frequency change ratio relative to a temperature change is l X 10 or so. In addition, the cutoff characteristic of the filter is also excellent, as will be seen from the aforementioned loss characteristic.

The cut-off characteristic can be enhanced by further connecting the planar tuning forks in side-by-side relation, as exemplified in FIG. 23. In the figure a planar tuning fork 103 identical with the forks 101 and 102 is interposed therebetween, and these tuning forks may be produced by punching of a sheet metal. In this case all the planar tuning forks are coupled integral with the support 113 through a coupling portion 106, extending from a base portion 104 of the intermediate tuning fork 103, while leaving out he coupling portions 106 and 106, of the other planar tuning forks depicted in FIG. 17. In order to couple the three planar tuning forks, coupling beams 109, and 109 are fixedly disposed between the vibratory reeds of the intermediate planar tuning fork 103 and the inner ones of the adjacent tuning forks, thus providing a unitary structure of the planar tuning forks. With the three planar tuning forks 101, 102 and 103 being coupled together in side-by-side relation, a filter of excellent selectivity can be obtained. It is possible to connect more planar tuning forks in side-by-side relation.

In the above example, the planar tuning forks are arranged with their vibratory reeds lying substantially in one plane but such arrangement is not always necessary. It is sufficient only to dispose a pair of vibratory reeds of at least one planar tuning fork in one plane. For example, it is possible that a plurality of planar tuning forks are assembled together by suitable coupling member in a manner to arrange their pairs of vibratory reeds one over another in spaced relation, as exemplified in FIG. 24. In the figure, four planar tuning forks 301, 302, 303 and 304 each having their two vibratory reeds in one plane are employed and these planar tuning forks are assembled together to arrange their pairs of vibratory reeds one over another in opposed and moderately spaced relation, and base portions 304, 304,, 304 and 304 of the planar tuning forks 301, 302, 303 and 304 are extended in a direction remote from the vibratory reeds. The extended base portions are respectively put between block members 30b, 30b,, 30b, and 30b; and bonded together, and coupling members 309,, 309 and 309 are each interposed between adjacent planar tuning forks at a place on the base portions 304, 304,, 304 and 304 or on the vibratory reeds close to the portion, thus interconnecting the planar tuning forks. In this case it is preferred that the planar tuning forks are each produced by punching of a sheet metal and assembled together so as to ensure uniformity of their temperature characteristic. Also in this case piezoelectric elements are mounted on the vibratory reeds of the uppermost and lowermost tuning forks.

In the examples above described with FIGS. 17, 23 and 24 the filters employ the plate-like planar tuning forks and hence they can be miniaturized in construction. Further, the planar tuning forks of high precision and uniform characteristics can be mass produced by punching process to ensure the fabrication of excellent filters. The use of the photoetching techniques employed in the manufacture of semiconductors allows ease in the production of small-sized planar tuning forks, which leads to further miniaturization of the filters.

In the above examples the piezoelectric elements are used as driving and detecting elements of the filters but they may be replaced with electromagnetic or electrostatic elements. It is of course possible to employ these three electromechanical transducer elements in combination.

FIGS. 25 and 26 illustrates a frequency selector unit which is applicable to an alarm device, a frequency analyzer or the like and in which a plurality of planar tuning forks of this invention are employed and signals are selectively picked up according to their frequencies from many input signals of different frequencies.

In the figure reference numerals 401, 402, 403, 40n respectively designate planar tuning forks such as exemplified in FIG. 2. The tuning forks 401, 402, 40n are respectively provided with a pair of vibratory reeds 401a and 401b, 402a and 402b, 40m and 40nb, in exactly the same manner as in the foregoing examples. All the planar tuning forks 401, 402, 40n are assembled together as a unitary structure at their base portions 404,, 404 404,, through coupling portions 409,, 409 409,, in such a manner that their vibratory reeds 401a, 401b, 40na, 40nb may lie in the same plane at certain intervals. Free ends of coupling portions 406,, 406 406,, extending from the central portions of the base portions of the tuning forks in a direction opposite to the vibratory reeds are bent substantially at right angles and are secured to a support 413 parallel to a base plate 406 which support 413 extends substantially in parallel with the vibratory reeds. Further, electromechanical transducer elements such, for example, as piezoelectric elements 4015a and 4015b, 4025a and 4025b, 40n5a and 40n5b are deposited, by means of an adhesive binder, on the vibratory reeds 401a and 401b, 402a and 402b, 40m and 40nb ofthe planar tuning forks 401,402, 4011 in proximity to the base portions 404,, 404 40411 thereof. The planar tuning forks 401, 402, 403, 40n are difierent in length of the vibratory reeds so as to obtain different resonance frequencies. The resonance frequencies f of the planar tuning forks are given by the aforementioned equation l as in the foregoing examples:

f= L L 21r l /fi p In FIG. 25 the planar tuning forks 401, 402, 403, 40n are adapted such that their resonance frequencies f,,, f f f, gradually lower. Namely, the lengths of the vibratory reeds of the planar tuning forks are rendered sequentially greater. In each tuning fork the width a of its vibratory reeds is selected greater than the space b between the reeds, for example, a /2 z b With b being greater than a torsional vibration is yielded in the base portion to change the resonance frequency f and hence a stable resonance frequency cannot be obtained. Further, the length 0,, of the base portion in the lengthwise direction of the vibratory reeds is selected greater than the width a for example, 1.5a, c, When c, is smaller than a vibration of the vibratory reeds is much transmitted from the coupling portion to the support to cause loss. As in the foregoing examples, the vibratory reeds of each planar tuning fork vibrate in anti-phase relation to each other at right angles to their plane. That is, they vibrate in such a manner that they first go away from their plane in opposite directions and then back to the plane.

In FIG. 25 the coupling portions 409,, 409 409,, are formed contiguous to adjacent ones at the ends of the base portions of the planar tuning forks remote from the vibratory reeds but coupling members may be formed contiguous directly to the vibratory reeds in proximity to the base por tions thereof. These planar tuning forks, their coupling portions and the support may be produced by punching process of a sheet of a resilient alloyed metal such as ELINVAR. That is, they are produced successively from one material without interruption. Where the piezoelectric elements of PZT are employed as the electromechanical transducer elements, it is preferred to locate them on the vibratory reeds of the planar tuning forks in close but definitely spaced relation to the base portions so as to obtain resonance frequencies nearly equal to calculated values. Further, it is also preferred to form the coupling portions 409,, 409 409 409,, as small as possible to ensure the elimination of interference between adjacent planar tuning forks. The support 413 for mounting the planar tuning forks on the same base plate 406 may be formed 

1. A mechanical vibrator comprising two U-Shaped mechanical vibrators each having a pair of strip-like vibratory reeds of substantially the same configuration and a base portion coupling together the pair of vibratoRy reeds at one end as a unitary structure, the lengths of said base portions of said two U-Shaped mechanical vibrators in the lengthwise direction of said vibratory reeds being referred to as c1 and c2 respectfully, another base portion coupling together said two U-Shaped vibrators at their respective base portions, in which the width a1 of the pair of vibratory reeds of one of said U-Shaped mechanical vibrators is selected fully greater than the thickness thereof, the width a2 of the pair vibratory reeds of the other said U-Shaped mechanical vibrators is selected fully greater than the thickness thereof, the pair of vibratory reeds of each said U-Shaped mechanical vibrator being arranged in spaced relation with distances b1 and b2 respectfully between the reeds and in parallel relation to each other with their surfaces lying in substantially the same plane, the widths a1 and a2 being selected greater than the distances b1 and b2 respectively, the lengths c1 and c2 being selected greater than the widths a1 and a2 respectively, the vibratory reeds effective to vibrate in antiphase relation to each other at right angles to the plane which includes their surfaces, and the length of the vibratory reeds of one of said two U-Shaped mechanical vibrators being longer than the vibrating reeds of the other U-Shaped mechanical vibrator, two plate like coupling portions each fixed to said base portion of each U-Shaped mechanical vibrator at its one end remote from said strip-like vibratory reeds thereof and extending therefrom in parallel with each other with its surface lying in substantially said same plane and then being bent to the direction substantially perpendicular to said same plane, and a support to which the free ends of said two plate like coupling portions are secured, whereby the amount of the vibration energy of the mechanical vibrators transmitted to said support is made as small as possible.
 2. A mechanical vibrator as claimed in claim 1, wherein the width a1 is selected to be equal the width a2.
 3. A mechanical vibrator as claimed in claim 1, wherein a separate electromechanical transducer element is mounted on each of said vibratory reeds of each of said U-Shaped mechanical vibrator to convert the mechanical vibrations of said reeds into electrical signals.
 4. A mechanical vibrator as claimed in claim 1, wherein said other base portion coupling together said two U-Shaped mechanical vibrators has provided therein a slit between said base portions of said two U-Shaped mechanical vibrators.
 5. A mechanical vibrator as claimed in claim 1, in which the depth of said slit from a line joining said base portions of said vibratory reeds is selected sufficiently long to permit the tuning forks to function independently from each other. 